In order to stimulate and more effectively produce hydrocarbons from downhole formations, especially formations with low porosity and/or low permeability, induced fracturing (called “frac operations”, “hydraulic fracturing”, or simply “fracing”) of the hydrocarbon-bearing formations has been a commonly used technique. In a typical frac operation, fluids are pumped downhole under high pressure, causing the formations to fracture around the borehole, creating high permeability conduits that promote the flow of the hydrocarbons into the borehole. These frac operations can be conducted in horizontal and deviated, as well as vertical, boreholes, and in either intervals of uncased wells, or in cased wells through perforations.
In cased boreholes in vertical wells, for example, the high pressure fluids exit the borehole via perforations through the casing and surrounding cement, and cause the formations to fracture, usually in thin, generally vertical sheet-like fractures in the deeper formations in which oil and gas are commonly found. These induced fractures generally extend laterally a considerable distance out from the wellbore into the surrounding formations, and extend vertically until the fracture reaches a formation that is not easily fractured above and/or below the desired frac interval. The directions of maximum and minimum horizontal stress within the formation determine the azimuthal orientation of the induced fractures. Normally, if the fluid, sometimes called slurry, pumped downhole does not contain solids that remain lodged in the fracture when the fluid pressure is relaxed, then the fracture re-closes, and most of the permeability conduit gain is lost.
These solids, called proppants, are generally composed of sand grains or ceramic particulates, and the fluid used to pump these solids downhole is usually designed to be sufficiently viscous such that the proppant particulates remain entrained in the fluid as it moves downhole and out into the induced fractures. Prior to producing the fractured formations, materials called “breakers” can be pumped downhole in the frac fluid slurry to reduce the viscosity of the frac fluid after a desired time delay to enable these fluids to be removed from the fractures during production.
The proppants can also be placed in the induced fractures with a low viscosity fluid in fracturing operations referred to as “water fracs” or “slick water fracs”. The fracturing fluid in water fracs is water with little or no polymer or other additives. Water fracs are advantageous because of the lower cost of the fluid used. Also when using cross-linked polymers, it is essential that the breakers be effective or the fluid cannot be recovered from the fracture, effectively restricting flow of formation fluids and thus reducing fracture permeability. Water fracs, because the fluid is not cross-linked, do not rely on the effectiveness of breakers. However, like the cross-linked polymers, water from the fracturing fluid can accumulate in the fracture, thereby reducing the permeability of gas or oil through the fracture.
The proppant placed into the fracture oftentimes forms a proppant pack permeable to water and/or hydrocarbons produced from the surrounding subterranean formation. Over time, the water and/or hydrocarbon production can lead to deposition of barium sulfate scale, paraffins, waxes and the like onto the proppant pack. This deposition can inhibit fluid flow and reduce well performance by effectively “choking off” the fracture in the area of deposition.
There is a need, therefore, for proppant particulates that promote the return of fracturing fluid and cross-linked polymers to the surface. There is also a need for proppant particulates that can limit deposition of scale, paraffins, waxes, and the like in order to maximize fracture permeability.